1. Technical Field
The present invention relates to a device for measuring the in-plane high-frequency magnetic field generated by a microwave-assisted magnetic head.
2. Background
In the past, surface recording density has been notably increased in magnetic recording media, such as hard disks or the like, by improvements, for example, of finer magnetic particles that configure a magnetic recording layer, of materials and finer head processing. Furthermore, recently magnetic recording and reproducing apparatuses have been commercialized with perpendicular magnetic recording methods in which the surface recording density is further improved by magnetizing the recording layer in a direction perpendicular to the surface of the magnetic recording medium. Further improvements in the surface recording density are anticipated in the future.
On the other hand, it is preferable to use as recording materials magnetic particles with large magnetic anisotropic energy Ku (magnetic anisotropic magnetic field Hk) and large coercive force Hc because thermal fluctuation of the recording magnetization tends to occur in correspondence to the greater fineness of recording bits and magnetic particles.
However, when magnetic particles with large magnetic anisotropic energy Ku are used as recording layer materials, the coercive force Hc of the recording layer becomes a large value of, for example, 4 kOe or more. When accomplishing saturation magnetic recording, it is generally said that a recording magnetic field of at least twice as large as the coercive force is necessary. Therefore, with the performance of conventional magnetic heads, cases arose in which it was extremely difficult to achieve saturation magnetization of the recording layer. In other words, there were cases in which recording and erasing magnetic data were difficult.
Magnetic recording of data onto a magnetic recording medium is accomplished by a perpendicular recording magnetic field generated from the tip of the main magnetic pole of the magnetic head. The perpendicular recording magnetic field is generated by applying a current to a main coil positioned adjacent to the main magnetic pole. One method has been studied in order to greatly reduce the perpendicular recording magnetic field that is required for causing the magnetization reversal. The method is conducted by overlapping alternating magnetic fields in an in-plane direction in the microwave band equal to or close to the ferromagnetic resonant frequency of the medium recording layer to a perpendicular recording magnetic field that induces such a magnetization reversal. This assisted recording method is known as microwave assisted magnetic recording (MAMR), and its efficacy has been verified experimentally.
Two methods have primarily been proposed for MAMR. One is a method in which a spin torque oscillator (STO) made of multiple layers of magnetic thin film is formed in a gap between the main magnetic pole and the auxiliary magnetic pole of the magnetic head and a microwave magnetic field in the in-plane direction is generated by driving a bias current and causing the STO to oscillate, for example as noted in reference document 1 (J. Zhu et al; IEEE TRANSACTION ON MAGNETICS, Vol. 44, No. 1, p. 125) (this is sometimes called a self-excited type).
The other is a method in which a supplementary coil is prepared in and/or in the vicinity of the gap between the main magnetic pole and the auxiliary magnetic pole of the magnetic head, and an in-plane alternating magnetic field is generated by driving the alternating current of the microwave band in the supplementary coil, for example as noted in reference document 2 (Japanese Laid-Open Application Publication No. 2007-299460 (sometimes called an induced type).
When considering mass production and commercialization of this type of magnetic head, the high frequency in-plane magnetic field intensities produced by the microwave-assisted magnetic heads must each be precisely measured in order to secure the reliability of the device. Therefore, it is necessary to develop a highly sensitive, low-cost property measuring apparatus.
However, the following significant technical issues are faced in developing this device.    (1) In both self-excited and induced types, a gap between the main magnetic pole and the auxiliary magnetic pole, and another gap in the vicinity thereof, where the in-plane high-frequency magnetic field is generated, are both assumed to be around 30 nm at the largest, so the in-plane high-frequency magnetic field is generated from an extremely tiny region.    (2) In order to realize the microwave-assisted effect, it is considered that a large in-plane high-frequency magnetic field is necessary, for example 2 kOe or greater. Such an in-plane high-frequency magnetic field is required.    (3) The frequency of the in-plane high-frequency magnetic field is the same as or close to the ferromagnetic resonant frequency of the recording layer of the magnetic recording medium that is the target of recording. The value is high because it is said that the value is generally in the range of 10 GHz to 40 GHz.
On the other hand, as a method for measuring the recording magnetic field of magnetic heads used in conventional longitudinal recording, a method has been proposed in which magnetic sensors, specifically a giant magnetoresistive (GMR) heads, are positioned opposite to a flying surface of the magnetic head (see Japanese Laid-Open Application Publication No. 2009-301610).
However, in the details of the proposal in the Japanese publication, the frequency of the recording drive current is around 20 to 700 MHz. This frequency is in a completely different frequency band from the 10 GHz to 40 GHz frequency that the microwave-assisted magnetic head should manifest.
In addition, when a GMR head is used as a measurement sensor, element resistance is low and output is small, so reliable measurements are extremely difficult to obtain only by adjacent positioning because the S/N ratio of the measured signal cannot be guaranteed.
On the other hand, when a tunneling magnetoresistive (TMR) head having much higher generated output than the GMR head is used as a measurement sensor, it is considered that reliable measurement is possible because the element resistance and its output are high so that the S/N ratio of the measurement signal can be guaranteed. However, with a TMR element, the intensity of the external magnetic field that can linearly respond (make a linear response) is at most several tens of Oe. Therefore, further engineering is necessary for measuring in-plane high-frequency magnetic fields generated by microwave-assisted magnetic heads, the in-plane high-frequency magnetic fields being considered around 2 kOe or more. Furthermore, in a TMR element, there is a difficulty to obtain the desired S/N ratio because high-frequency characteristics are poor since the impedance with respect to element resistance is large, and responsiveness corresponding to frequencies of 10 GHz or more generated by the microwave-assisted magnetic head is inadequate.
The present invention was invented in light of these actual circumstances, and it is an objective of this invention to provide a measuring apparatus that can reliably and precisely measure the in-plane high-frequency magnetic field generated by a microwave-assisted magnetic head.
Such a measuring apparatus can assure high density recording and improved recording quality, and can contribute to simplifying, reducing the cost of and increasing the throughput of shipping inspections.